So far I've looked at the following possibilities: 127mm Apex Max, a used 4" F11-12 achromatic refractor, a 6" F8 reflector, and a used 8" F6 reflector. As I want the scope for double star use I am thinking that I want nice bright contrasty views so this kind of turns thumbs down on the 127mm Apex, as it has a large central obstruction. If I could locate a 4" F11-F12 achromatic refractor with a mount and get all of that shipped to me for $700 I think I'd be very pleased. And, I should think (if need be) I could buy the OTA and the mount separately. I also looked at achromats in the shorter focal lengths around 4" of aperture, but I want the apochromatic view that I could get in a long tube refractor! Otherwise I am considering a reflector with minimal central obstruction and a long focal length to provide good double star results. Parks makes 6" F6 reflector with Alt/AZ pier mount, but I am thinking that F8 would be better for the double stars. Star gazer Steve has a nice 6" F8 kit (it is in my budget too) but I am not sure about availability. And, if it is available I still kind of lean towards a used long tube refractor. I have considered the Orion XT8, but it doesn't have the fit and finish I'd prefer, I'd like something a bit nicer. What is my best buy new or used (with shipping) for $700 or less?

You might consider the Celestron Omni 102XLT. This is a 4" f9.8 (1000mm f/l) achromat refractor on a CG4 Mount. Nonmotorized it will run around $450, motorized (RA & DEC) will run just under $600. Nice Scope, great views. Tracks great. You could get its bigger brother (the 120XLT, same mount) for about $100 more. Love my 102XLT for planets, double stars, clusters....

Here's the deal with double stars. They are highly sensitive to seeing conditions. Though a larger instrument may have greater resolving power on paper, how often will your seeing allow *any* instrument to split sub-arc-second doubles? Also, the larger the aperture, the more sensitive to poor seeing.

With even a 60mm refractor there are literally thousands of interesting doubles within reach of the aperture, and because it is small enough NOT to be affected much by seeing, you'll be able to hit the little scope's theoretical limits on most nights. The same would not be true of a significantly larger aperture.

I think the erroneous assumption inherent in your post is that you have to have larger aperture to enjoy double stars. That's simply not the case. Also, particularly on very unequal magnitude doubles, a smaller scope that controls contrast is superior to a larger scope that has a lot of scatter.

IMO you could have an absolute blast on doubles with a quality 3" achromat on a simple alt-az mount. I like this set-up:

$399 (maybe less if you shop around), and that leaves money for a copy of the Cambridge Double Star Atlas and a couple of well-chosen focal lengths of the upcoming re-introduced University HD Orthoscopic eyepiece line.

So.. there are many ways to observe double stars, as Jim said, double stars are everywhere, what are you hoping to do, are you looking to get nice views of relatively easier double stars? Or are you looking for a rig that will push the limits of what is possible, something that demands more of the sky, more of your setup and care but that will, when everything is dialed in, split 1.0 arc-second double wide open and get those sub-arcsecond doubles cleanly?

If clean and pretty and relative ease are what you are looking for, then a I think smaller scope, is in order, probably a refractor. A used ED-100 for $400 plus either a simple EQ mount with drives or an alt-az mount would provide aesthetically pleasing splits and be capable of splitting most of the popular doubles.

If pushing the limits are the goal, a 10 inch or even 12 inch Dobsonian with a good fan will allow you to work the tightest doubles but it will be more demanding of the seeing and will require more attention on your part.

I would not discount the 5 inch Mak. I am not a big fan of Maks but for this application, the CO is not such a big deal and their compact size makes them easy to use...

Hi AstroOne: From the look of you picture you already have a pretty nice rig. Do you find it unsatisfactory or are you looking for a different observing experience? I suspect an 3-4" F15 acromatic refractor would give you the experience you seek if, in fact, that is what you are looking for; why go for F11 when F15 is the usual f-ratio for classic refractors.

Good luck finding a 4" f/15 achromat and a suitable mount for $700. In fact, good luck finding a new or used 3" f/15 with modern 1.25" focuser (rather than a 0.965" unit), at all.

A fine, well-accesoriezed 3" f/11.4 is available bundled with an excellent alt-az mount from stock, shipped to your door in a few days from time of order. So I think the attraction of a 3" f/11.4 is availability. I'd rather have an f/15 if a new one was available from stock with a 1.25" focuser, but I know of no new option like that. The used ones usually have 0.965" focusers which are not convenient for most folks.

I had a Vixen A80MF for a while. It's an 80mm F/11 achromat with a typical Synta focuser. It's a fun scope but not one I would choose just for viewing double stars. I like 80mm F/11 achromats but they do show some chromatic aberration and it does get in the way with targets like Rigel or Delta Cygni, faint companions that are doable.

Like any Synta rack and pinion focuser, with some effort, reworking it, it can be a very reasonable focuser but stock, its going to be stiff with some rocking.

If one is looking for an 80mm F/11 and willing to wait for a used one, the Meade 310 and 320's are the best ones I have used. I have two of them, they are keepers. The Mizar mounts are similar to the Vixen Polaris's, very solid and fully geared, the EQ version is capable of being used as an alt-az mount. The OTA's are quite special, they appear to be identical except for the color to the 80mm Astro-Physics guide scopes... the focuser is a step up from the Vixens..

But I don't see an 80mm as something I would choose for a double star specific scope and if I were, I would be looking at an 80mm F/7 ED/APO with FPL-53 glass or maybe an old 76 mm F/15 on a modern, stable mount. The 80mm ed/apo is the one I choose when I am going after doubles with an 80mm.

But I do think 80mm is not enough for a dedicated double star rig and the ED-100's come up quite frequently on Astromart of $400, same focal length and Vixen 80mm F/11 but a better all around performer.

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An 8 inch f/6 Dob with a good mirror will outperform all of your other choices. Mine constantly beats 4 and 5 inch Apo's. The fit and finish of an 8 inch econodob can be improved by upgrading the mount to baltic birch. This also reduces the weight.

Jim, thanks for the excellent post, the 3.2" Vixen is definitely in the arena for consideration. I got the idea of focal lengths from Philip Harrington's, "Star Ware." Per his book the 3.2" should be somewhere longer than a FL of 9.22; the F11.4 certainly meets that criteria. A 4" APO used, possibly the Meade married up to the mount that goes with the recommended Vixen is something else for me to consider.

Jon, Thank you for the post and the question on what types of doubles I want to observe. The set up Jim mentioned is definitely in my thinking cap but I do think I would like a bit more aperture so a 4" used Apo is quite appealing too. I am familiar with the close E & F doubles of the trapezium and also the double double in Lyra. I've scene them numerous times through my 15" scope and hope that the scope I am now considering will will also split these doubles, if not though I can live through it (LOL). Many years ago I had a 4.25" F10 Criterion reflector it did a fine job cleanly splitting the double double. When I go out with my 15" Obsession I often check the seeing conditions against the dbl dbl. I am more after the easier and scenic doubles than the really closer ones. My house sits on the edge of a yellow zone, it is one zone less dark here. I hope to use the scope mostly in my back yard, but I also have a yellow zone site at about 2,000' higher elevation just twenty minutes away, so I may go there some too. When the moon is new I head out with the bigger scope to visit grey or black zone sites. This smaller scope will be good for grab and go use. I do lean towards the 4" though because it will show more of Saturn and Jupiter and if I am out it will be impossible for me not to take a look at them too. Best Regards,Steve

Hi Ed, The F15 195mm sounds good but if I bought one I'd probably have to have a very substantial mount to support the long tube. With that in mind either the 3.2" Vixen or 4" used APO will probably work best. Thanks for the post,Steve

Before I bought my 15" scope I owned an Orion XTi, it was optically good but seemed kind of cheap. It did a good job for me though, but I got aperture fever. At times I've wished I hadn't sold that scope, as it would do a pretty decent job as a second smaller scope. The majority of responders to my query have recommended refractors, nice to here a vote for a reflector, but I am leaning toward a refractor at this point. Regards,Steve

I am familiar with the close E & F doubles of the trapezium and also the double double in Lyra. I've scene them numerous times through my 15" scope and hope that the scope I am now considering will will also split these doubles, if not though I can live through it (LOL).

The double-double is a relatively easy split in a decent 80mm, the E-F stars are more challenging because they are somewhat wider, around 4 arc-seconds but 11th magnitude so they can be easily lost in poor seeing.

The Dawes limit for a 80mm is about 1.45 arc-seconds, a good 80mm can operate in this range when the seeing is excellent. For comparison, the Double-Double is 2.3 arc-seconds. Since you have a 15 inch Dob, a 3 or 4 inch refractor can make a good companion, not only are they handy but they can show wider fields of view. For that reason, I like mine small refractors fast and ED or APO allows for top notch performance at high powers.

Yup, the Double-double is a fairly easy target in a 60mm. Trapezium E and F, though, are not easy in a 3-incher. The are doable, provided that the scope is well-baffled and seeing cooperates. E and F are *much* easier in a 4" refractor; ~100-120x seems optimal. I find all unequal doubles doable in much smaller refractors than they are in larger scopes of other designs. I believe it has to do with the refractor's inherently better glare suppression. For example, I find Sirius B pretty easy in a quality 3" refractor at just under 100x. Read the double star forum posts about Sirius B. You'll see all kinds of crazy suggestions along the lines of it requiring 10" to 12" and high magnifications.

The thing about double stars is this: there are literally thousands of them visible in any sized telescope. Sure, a larger scope might let you split tighter doubles (but not always; just ask my club mates using 10" to 15" Dobs not being able to split Sirius when a 3" will do so), but there's always an equivalent aesthetic experience available in the smaller scope. In other words, you can find a double near the limits of a 3" scope that looks a lot like a dimmer pair at the limits of a 12" scope. In fact, I think doubles are almost more fun in a small scope less hampered by seeing. That said, 4" is a good compromise if you can swing it within your budget.

I believe it has to do with the refractor's inherently better glare suppression. For example, I find Sirius B pretty easy in a quality 3" refractor at just under 100x. Read the double star forum posts about Sirius B. You'll see all kinds of crazy suggestions along the lines of it requiring 10" to 12" and high magnifications.

Sirius B is still widening... getting easier, 10 years ago it was 6 arc-seconds instead of 10 and much more difficult. When was the last time you split Sirius in an 80mm?

Good luck finding a 4" f/15 achromat and a suitable mount for $700. In fact, good luck finding a new or used 3" f/15 with modern 1.25" focuser (rather than a 0.965" unit), at all.

Hi Jim,

Far be it for me to reveal something of which you are not aware, but here is a brand spanking new 101mm f/15 with what to me looks like a 1.25" or 2" focuser: link. The cost might be a problem, though...